Fatiguing stimulation increases curvature of the force-velocity relation in isolated fast-twitch and slow-twitch rat muscles

Abstract

In skeletal muscles, the ability to generate power is reduced during fatigue. In isolated muscles, maximal power can be calculated from the force-velocity relationship. This relationship is well described by the Hill equation, which contains three parameters: (1) maximal isometric force, (2) maximum contraction velocity and (3) curvature. Here, we investigated the hypothesis that a fatigue-induced loss of power is associated with changes in curvature of the force-velocity curve in slow-twitch muscles but not in fast-twitch muscles during the development of fatigue. Isolated rat soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscles were incubated in Krebs-Ringer solution at 30°C and stimulated electrically at 60 Hz (soleus) and 150 Hz (EDL) to perform a series of concentric contractions to fatigue. Force-velocity data were fitted to the Hill equation, and curvature was determined as the ratio of the curve parameters a/F 0 (inversely related to curvature). At the end of the fatiguing protocol, maximal power decreased by 58±5% in the soleus and 69±4% in the EDL compared with initial values in non-fatigued muscles. At the end of the fatiguing sequence, curvature increased as judged from the decrease in a/F 0 by 81±20% in the soleus and by 31±12% in the EDL. However, during the initial phases of fatiguing stimulation, we observed a small decrease in curvature in the EDL, but not in the soleus, which may be a result of post-activation potentiation. In conclusion, fatigue-induced loss of power is strongly associated with an increased curvature of the force-velocity relationship, particularly in slow-twitch muscles.

Differential changes in muscle architecture and neuromuscular fatigability induced by isometric resistance training at short and long muscle-tendon unit lengths

“…PA has important physiological effects on the force-velocity relationship and thus peak power (20). Considering that an increase in curvature of the force-velocity relationship is induced by fatigue (41,44), PA is also expected to influence neuromuscular fatigue resistance. PA relates to muscle strength (14,70), owing to the muscle size-strength association (3,4,31).…”

Are Force Enhancement after Stretch and Muscle Fatigue Due to Effects of Elevated Inorganic Phosphate and Low Calcium on Cross Bridge Kinetics?

“…The proportional reductions in force, stiffness and extra force during a stretch at pCa 5.5 suggest that reducing Ca 2+ results in an increased proportion of detached cross bridges, as indeed seen in the model calculations. Increased curvature (decreased a/Po) contributes to the decline in power during fatigue [4][5][6], and it has been suggested that this is a consequence of reduced intracellular Ca 2+ [2]. However, pCa 5.5 was associated with an increased a/Po, as also seen before in single fibres [14] and isolated muscle preparations [26], rather than a decreased a/Po.…”

“…However, reduced cross bridge recruitment cannot be the whole story, as the loss of power is more than proportional to the loss of force [2]. Indeed, during the development of fatigue, the maximal shortening velocity also decreases and the curvature of the force-velocity relationship increases (reduced a/Po), both contributing to the loss of power during a series of repeated contractions [4][5][6].…”

Differential contributions of fatigue‐induced strength loss and slowing of angular velocity to power loss following repeated maximal shortening contractions

“…Consequently, the relative contributions of reductions in angular velocity and dynamic torque on power loss varied depending on the timepoint: The relative contribution of fatigue‐related reduction in dynamic torque on power loss was greater immediately following the task, and lower throughout recovery than the corresponding decrease in angular velocity. These results appear to be affected by the increase in the curvature of the force‐velocity relationship due to fatigue, influencing power loss (Jones, ; Kristensen et al, ).…”

“…As suggested previously (Cheng & Rice, ), fatigue‐induced power loss is only partially explained by the corresponding reduction in isometric MVC torque, and thus, assessments of shortening velocity are useful in identifying the underlying factors contributing performance fatigability (Cheng & Rice, ; Dalton et al, ). Furthermore, considering that fatigue‐induced power loss is partly due to an increase in curvature of the force‐velocity relationship of a muscle induced by fatigue (Jones, ; Kristensen, Nielsen, Pedersen, & Overgaard, ), a fatigue‐induced decline in dynamic torque can also be an important parameter for understanding the mechanism of neuromuscular fatigue and recovery. Recently, a positive relationship has been reported between fatigue‐related decreases in angular velocity, dynamic torque and power loss (Lanning et al, ; Wallace et al, ) using isotonic contractions of the plantar flexors; however, the difference in the extent to which the reductions in angular velocity and dynamic torque can explain power loss following a fatiguing task has not been statistically quantified.…”